1. Field of the Invention
The present invention relates to a fluidized bed type pulverization/classification apparatus for manufacturing a powder and a method for manufacturing a powder using the pulverization/classification apparatus.
Particularly, the present invention relates to a fluidized bed type pulverization/classification apparatus for pulverizing and classifying a toner constituent mixture which includes at least a binder resin and a colorant and which has been subjected to melt-kneading, cooling and crushing treatments, to prepare a toner having a relatively small particle diameter. In addition, the present invention also relates to a method for manufacturing a toner having a relatively small particle diameter using the fluidized bed type pulverization/classification apparatus.
2. Discussion of the Background
Conventional fluidized bed type pulverization/classification apparatuses typically have a cylindrical shape and include a vessel. In addition, plural air nozzles are provided on the lower portions of the inner wall of the vessel to discharge a high pressure jet air. In the cylindrical vessel, particles of a toner raw material (i.e., a toner constituent mixture) are suspended by the high pressure jet air so as to be collided to each other. In order to efficiently perform pulverization while preventing excessive pulverization, the pulverized toner raw material needs to be rapidly fed to a classifier so that the toner particles have a small particle diameter in a proper diameter range.
FIG. 1 illustrates a conventional fluidized bed type pulverization/classification apparatus which includes a pulverizer and a classifier.
Referring to FIG. 1, particles 2 (hereinafter referred to as raw material particles 2) of a toner raw material which are fed from a raw material feeder 7 into a cylindrical vessel 1 are accelerated by high pressure jet air fed by an air feeder 6 and discharged from plural air nozzles 3. The raw material particles 2 collide with each other at a crossing point of the jet air streams, resulting in pulverization of the particles 2. The raw material particles 2 stay in the cylindrical vessel 1 for a predetermined time while circling therein. After the pulverization is repeated, the pulverized raw material particles 2 are fed by upward current to a classification rotor 4 of the classifier, which is provided on an upper portion of the cylindrical vessel 1, so as to be classified into particles having a particle diameter in a proper particle diameter range and coarse particles (fine particles are discharged using a blower). The particles having a particle diameter in a proper diameter range are used as a toner which is a final product. The coarse particles are fed again to the cylindrical vessel 1 to be further subjected to the pulverization treatment. By repeating these operations, the raw material particles 2 are converted to the final toner product.
Various pulverization methods using a cylindrical vessel have been proposed in attempting to solve various problems.
Unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 05-146704 discloses a method in which the feeding quantity of raw material particles is controlled in attempting to avoid variation in the classification accuracy.
JP-A 06-285386 discloses a device which can measure the concentration of particles in the air in a pulverizer in attempting to easily operate the pulverizer.
JP-A 11-70340 discloses a technique of minimizing turbulence loss in a vessel caused by air flow and raw material particles, which have different flow rates.
In addition, JP-As 2-294660, 2-294661, 2-294662 and 2-294663 disclose methods for manufacturing a toner having a general particle diameter using a pulverization/classification method in which a first classifier and a second classifier are used as a classifier. When a classification operation is performed for a long period of time using such a pulverizer, soft resin components included in toner constituents tends to adhere to a particle discharging tube of the second classifier etc., resulting in formation of coarse toner particles, and thereby the coarse toner particles are included in the resultant final toner product. In attempting to avoid adhesion of such soft resin components, the applications disclose a method which uses a particle discharging tube, at least a part of which is made of a fluorine-containing resin.
Recently, there is a desire for high resolution images in electrophotography. Therefore, a strong need exists for a toner having a small particle diameter. Namely, a need exists for a toner having a small weight average particle diameter of from about 4.5 μm to 8.0 μm.
In such a case, when the particle diameter of pulverized raw material particles is small, the state of the pulverized raw material particles in a pulverizer is different from that in the conventional toner case. Therefore it is necessary to change the operation conditions of the pulverizer used.
Provided that the true specific gravity of toner raw material particles is 1.2 and the particle diameter of the particles changes from 9.0 μm to 7.5 μm and 6.0 μm, the number of the particles increases from 2.7 billion pieces per gram to 4.7 billion and 9.3 billion pieces per gram, respectively. In this case, if the powder concentration is 0.05% in a pulverizer, the center distance of the particles decreases from 90 μm to 75 μm and 60 μm, respectively. In addition, the Brownian motion of the particles changes from 2 μm/sec to 2.52 μm/sec and 3 μm/sec, respectively.
When a toner powder having such a small particle diameter is manufactured using a fluidized bed type pulverization/classification method, the following problems tend to occur:    (1) The pulverization ability of the pulverizer used deteriorates;    (2) coarse particles (having a particle diameter not less than about 16 μm) are included in the final toner product; and    (3) the content of super fine particles (having a particle diameter not greater than about 4 μm) in the final toner product increases.
The content of such coarse particles and super fine particles in the final toner product prepared by conventional fluidized bed pulverization/classification methods is considerably high.
When the content of super fine particles in a pulverized product increases, aggregates thereof tend to be formed. In this case, aggregates are considered to be coarse particles and therefore are re-used, resulting in increase of fine particles, and thereby the image qualities deteriorate. In order to avoid such a problem, classification is severely performed such that the classification particle range is narrowed. In this case, another problem occurs in that the yield of a final toner product having a particle diameter in a desired range seriously decreases. In addition, a developer including super fine particles and coarse particles in a large amount does not have a stable charge quantity, and thereby the resultant images have a low image density.
Namely, inclusion of super fine particles and coarse particles in a toner in a large amount affects the charge quantity of the toner. When raw materials are excessively pulverized in attempting to avoid inclusion of coarse particles in a toner, the resultant toner images have a background fouling problem, resulting in deterioration of image qualities. To the contrary, an insufficiently pulverized toner causes a poor image transfer problem, also resulting in deterioration of image qualities.
Such problems specific to a toner having a small particle diameter and manufacturing problems thereof are newly recognized problems, and therefore a solution thereof has not yet been proposed.
Because of these reasons, a need exists for a method and apparatus capable of producing an electrophotographic toner having a small particle diameter in which super fine particles and coarse particles are included in a relatively small amount and which has a stable charge quantity and can produce high resolution images.